Step 2: Scales

Take a look at an actual photo of mystique, the one from the movies obviously. It will be your best inspiration to make your own. This is how I made my own:

Cut your plastic sheet in ovals, or other shape so you can attach them to each part of your body where mystique has scales, then put the plastic shape on a flat surface.Cut your foam sheets in the shape of mystique's scales.Glue them with hot glue to the plastic.

Instead of using foam board or if you are too lazy to cut one by one, you can use hot glue lines, blobs, or any shape you want directly on the plastic sheet, so when the hot glue dries the hot glue will be hard.

No need to paint the at this point, the ones in the pics are painted because they are post-halloween.

This is the first I'ble I've seen that includes instructions to "Now, take off your clothes.

Getting the color right would really be important as evidenced with the photo comparison to the smurfs. You don't want yours to be confused with that other costume.

I suppose depending on how daring a person is you could range from au natural to pasties to leotard(s) to several other bra type devices to cover the goodies. Then use lots of painted on latex scales to complete the camouflage.